Method for error reduction in lithography

ABSTRACT

The present invention relates to a method and a system for predicting and/or measuring and correcting geometrical errors in lithography using masks, such as large-area photomasks or reticles, and exposure stations, such as wafer steppers or projection aligners, printing the pattern of said masks on a workpiece, such as a display panel or a semiconductor wafer. A method to compensate for process variations when printing a pattern on a workpiece, including determining a two-dimensional CD profile in said pattern printed on said workpiece, generating a two-dimensional compensation file to equalize fluctuations in said two-dimensional CD-profile, and patterning a workpiece with said two-dimensional compensation file.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending application Ser.No. 09/979,148, filed on Nov. 20, 2001 and for which priority is claimedunder 35 U.S.C. § 120. Application Ser. No. 09/979,148 is the nationalphase of PCT International Application No. PCT/SE00/01030 filed on May22, 2000 under 35 U.S.C. § 371. This application also claims priority ofApplication No. 9901866-5 filed in Sweden on May 20, 1999 under 35U.S.C. § 119. The entire contents of each of the above-identifiedapplications are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to production and precision patterning ofphotomasks and use of such photomasks in microlithography, for examplein the production of flat panel displays and semiconductor circuits.Errors in the pattern on the display panel or semiconductor chip can beseparated into errors from systematic sources, from the interaction ofmaskblank and pattern with the equipment, and from random fluctuations.The invention relates to the reduction of these errors. In a differentsense the invention relates to the characterization of the photomasksubstrate and the equipment and processes used with a photomask, thestorage and retrieval of information obtained by such characterization,and the generation of corrections to be applied at the time of writingthe photomask in order to reduce imperfections in lithography usingphotomasks.

BACKGROUND

The development of semiconductor lithography has been exponential sincethe early 60ies and the produced features are getting smaller everysecond or third year, at the same time as the circuits get faster andmore complex. FIG. 1 shows an industry projection of the development forsome years forward. Of course the predictions are less certain thefarther into the future we look and nobody knows if the electronicsindustry will still be using transistors in the year 2020. For the next10 years the projections are more certain and the main uncertaintyrelates not to “How small?” but to “Exactly when?”.

The errors in lithography can broadly be classified as placement andsize errors, or “registration” and “critical dimension”, (“CD”) in thejargon of the trade. There is a more or less fixed relation between theerrors that can be allowed in the pattern and the size of the smallestfeatures in the pattern. A rule of thumb is that on the mask theplacement of figures has to be within 5% of the design rule and the sizeof the features should be within 2.5%. These are surprisingly smallnumbers, but have been justified by both theory and experiments. FIG. 1also shows the necessary registration and CD (size) control on the maskfor each year, assuming that 4× masks will continue to be used. It isseen that the errors are now 1999 in the low tens of nanometers and willin less than 15 years be ten times smaller. At the same time then chipswill be larger which means either larger masks or less reduction. Eitherway it will be difficult to achieve the needed pattern fidelity.

The invention devises a new general method to reduce errors in thelithography in order to achieve total errors that are consistent withthe projected lithography development. An important application is forthe reduction of clamping errors, being both an important error sourceand a good example of errors caused by the interaction of severalfactors.

Clamping Errors

When a glass plate is held it is deformed by the holding device and byits own weight. Furthermore, it can also be distorted by the built-instress in surface films deposited on it and by the patterning of saidfilms. Semiconductor masks are typically 152×152×6.25 mm and thepatterned area may be 127×127 mm. FIG. 2 b shows how the bending of aplate 201 under the force of gravitation causes the upper area of theglass to contract. When the plate is released, e.g. held vertically, itsprings back to its natural shape, shown in FIG. 2 a, and contractiondisappears. If a pattern was written on the plate while it was bent, thepattern will be stretched after relief. FIG. 3 shows a diagram with theresulting maximum error when a plate is supported along two oppositeedges, as in FIG. 2. Here, the expected lateral position error is shownas a function of the thickness of the plate and the size, i.e. thedistance between the two supported sides. The interesting conclusionfrom FIG. 3 is that the magnitude of errors that can result frominappropriate support of a glass plate is in order of magnitude largerthan what is allowable in a high-end mask. Point A shows a standardsemiconductor reticle 152×152×6.25 mm and the maximum deviation isaround 400 nm. Point B is the new standardised mask format 225×225×9 mmand, despite the fact that the glass plate is thicker, the deviation isabove 1 μm. Point C finally illustrates that for large-area masks theproblem is even worse: an 800 mm plate 8 mm gives a possible error of 60μm. It is also seen from FIG. 3 that increasing the thickness of theglass plate is a weak remedy. It is impossible to increase the thicknessof the 800 mm plate to bring the error down to 0.1 μm. The same is validfor the 225 mm mask in B: even a glass cube with the side 225 mm hasdeviations larger than 10 nm.

In conclusion FIG. 3 shows the magnitude of the gravitationaldeformation and it shows how everything becomes more difficult forlarger mask sizes.

Other Errors

Clamping deformation gives a placement or registration error. Anotherlarge source of placement errors in the finished product is thedistorsion in the exposure tool, be it a wafer stepper forsemiconductors or a projection aligner for display panels. Themaskwriter has stage errors, but these are usually well controlled aftercalibration to a xy metrology system, such as those made commerciallyavailable by Nikon and Leica. The placement can also be affected byprocessing, both because films deposited on or removed from a workpiecehave built-in stress and deform the workpiece, and because some processsteps cause a shrinkage or warping of the workpiece, e.g. hightemperature annealing steps. An important source of errors is thepellicle used on masks. Fitting the frame to the mask plate causes themask plate to bend.

Other effects make the size of the pattern features come out differentlyat different location on a mask or on a chip or display panel. There areseveral possible mechanisms behind this: uneven focus, non-uniformdeveloper agitation, non-uniform photoresist thickness, uneven chromeproperties on the mask and uneven film thickness on a wafer or panel,exposure dose variations in the exposure tools, effects of the timebetween the exposure and development or between resist coating andexposure and effect from non-perfect pre-exposure and post-exposurebaking procedures. Size errors also occur because of the basic imagingproperties of mask writer and the exposure station. In particular smallfeatures tend to come out too small due to finite resolution andfeatures are affected by the presence of other features in theneighbourhood due to stray exposure. These types of size errors are alsointimately coupled to shape errors, such as shortening of line ends androunding of corners. The exact details of the mask and wafer exposuretools also interact with the pattern and create for example grid snapeffects, and spurious pattern features.

Mix and Match

The term used in the mask industry is “registration” which really meansmisregistration from a reference grid, normally an ideal mathematicalgrid. In the past registration of the finished product to an idealmathematical grid has not been necessary. If all layers (approximately25 in a semiconductor chip and 6 in a TFT) are printed using the sametype of exposure station systematic and equal behaviour of the exposurestations will cancel, since every layer is distorted in the same way.

However, when resolution is pushed in order to achieve circuit speed andpacking density the cost of lithography is rising rapidly, both becauseof higher tool cost and because of more expensive masks. To makeproduction economical the display and chip manufacturers are trying notto use more sophisticated technology than needed for each layer, socalled mix-and-match. Different layers can be printed using differenttypes of exposure tools with different error characteristics.Furthermore the masks may be of different type, e.g. phase-shiftingmasks for one layer and standard binary masks on another layer. Thedifferent types of masks may require them to be written on differentmaskwriters.

The management of errors is made more complicated by the fact that theexposure tools for critical and non-critical layers may not even havethe same exposure field. FIG. 4 shows dies formed on a semiconductorwafer using tools with different fields. The critical layers like thetransistor layers are printed with a tool that has a field thataccommodates only one die, FIG. 4 a. Less critical layers, such as thetop metal layers, are printed with a different stepper having a largerfield and possibly a different mask reduction factor, FIG. 4 b. One orboth of the layers can be exposed by a step-and-scan tool, and if bothuse step and scan they may very well have the scan direction at rightangle to each other. Furthermore, it is anticipated that in the future asingle die will be exposed in two or more scanning strokes using a socalled stitching scanner, FIG. 4 c.

In the past the thinking has been that the masks should be as close toperfect, i.e. as close to the ideal mathematical grid, as possible. Thenthe masks for different layers can be written on different types ofmaskwriters or even by different mask making companies for best economyand logistics. When all masks are exposed on the same stepper, or onsteppers of the same type, the systematic errors in the stepper willlargely cancel. This breaks down in the mix-and-match scenario of FIG. 4a-c. The only straightforward way to resolve the complicated overlayproperties of layers printed in different ways is to make each layerprint an image that is close to the mathematically ideal.

The invention gives a method to predict the printing errors in aspecific exposure station and correct it beforehand by predistorting andprebiasing the pattern in the opposite sense. In this application isdescribed a practical method to manage the lithographic errors al theway to the finished product, and also in the presence of non-ideal maskblanks and clamping structures.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor predicting and correcting geometrical errors in lithography in orderto achieve an improved precision.

This object is achieved with a method according to the appended claims.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 gives a projection of future lithography and the requirements onplacement (registration) and size (CD).

FIG. 2 a shows a plate, e.g. a mask substrate. FIG. 2 b shows the sameplate supported at two sides and the contractive stress above and thetensile stress below the neutral layer which keeps its length when theplate is bent. FIG. 2 c shows the lateral displacement resulting from abent plate and how it is related to the dz/dx of the surface.

FIG. 3 shows the maximum lateral displacement in the arrangement of FIG.1 b as a function of plate size and thickness. The shaded areas show theresulting error.

FIG. 4 a-c shows dies formed on a semiconductor wafer using tools withdifferent fields, so called mix-and-match lithography.

FIG. 5 shows schematically a control method according to the invention.

FIG. 6 shows flow chart describing the development of models and how themodels are used to predict and correct errors.

FIG. 7 shows the control method according to FIG. 5 more in detail.

FIG. 8 shows how the plate and/or the pattern is affected by differenttypes of errors.

FIG. 9 shows a typical implementation of an error correction systemsuitable for the invention using both pattern modification and writinghardware control.

FIG. 10 is a schematic view of a system according to one embodiment ofthe invention.

FIG. 11 is a schematic flow chart of the method used in the systemaccording to FIG. 10.

FIG. 12 is a schematic view illustrating the correction in theembodiment in FIG. 5 in more detail.

FIG. 13 illustrates an SLM writer and a DUV stepper with a programmablemask according to another embodiment of the invention.

FIG. 14 a illustrates an example of a two-dimensional (2D) cylindricalCD distribution according to another embodiment of the invention.

FIG. 14 b illustrates an example of cylindrical CD distributionaccording to another embodiment of the invention.

FIG. 14 c illustrates an example of cylindrical CD distribution usingglobal dose compensation according to another embodiment of theinvention.

FIG. 14 d illustrates an example of a pre-programmed global dose mapimplementable using gray levels of an SLM or by utilizing dose controlin a laser according to another embodiment of the invention.

DESCRIPTION OF THE INVENTION

The invention is best described as a control system, such as is shown inFIG. 5. The pattern picks up errors 501, 502 of different kinds when itis converted from a design data file 503 and a mask blank 504 to a mask505 during a mask writing procedure 506, i.e. exposure, by means of amask writer 507. The mask is thereafter used to produce an electronicdevice 508. The pattern file describes what the chip or panel designerwants to see printed and any deviation is to him an error. One part ofthese errors is that systematic and other errors are different from onetime to another. The invention is based on the identification ofdifferent types of errors and the appropriate way to reduce each type.Errors that can be found in the output from the system and identified tobe recurring in a systematic fashion are reduced by feeding an inverseerror 509 back to the writing of the mask. This is the feed-back loop510 in FIG. 5. The feed-back can be pseudo-continuous, i.e. correctionsare made after each written mask, or intermittently. The error ismeasured by a measuring means 511, and a filter 512 is useful to keepthe feed-back from fluctuation with the noise component of the measurederror. The low-pass filter 512 can use a very simple procedure: use theaverage of five measurements over five days for setting the firstfeed-back, then change it only when the running average over fiveconsecutive measurement are outside of predetermined tolerance interval.On the other hand it can be based on more sophisticated statistics suchas time series estimations and feed back an appropriately filteredcorrection for each measurement of the output errors. More elaboratestatistics can be used to cancel slow changes in the properties of thetotal system without ever producing a rejected part, or to extract errorcomponents with different characteristics. A statistician skilled in theart can set up statistical process control (SPC) procedures that tunesthe system without interrupting the production flow.

In FIG. 12 the correction system provided in the embodiment describedabove with reference to FIG. 5 is illustrated in more detail. Input datais transferred to a data collection unit 1201 in the error reductionsystem 506. As will be described more in detail in the following, suchinput data could be one, or preferably several, and most preferably all,of the following: pattern (design) data, blank data mask writer data,exposure tool data, process data and metrology tool data. The input datais then forwarded to a data validation unit 1202, where the data isvalidated. The validated data is then transferred to error predictionunit 1203 comprising a model, for a statistical estimation of modelparameters. Correction data is then output as a position correction map1204 and/or a correction size map 1203, for correction of patternelement position and pattern element size, respectively.

The correction maps are forwarded to the mask writer 507. One or both ofthe correction maps could be forwarded to the data path 1206 in the maskwriter 507 for correction of the data provided from the pattern datafile, and thus correcting said distorsion. The altering of the inputdesign data could hereby preferably be made in at least one processor,and preferably in several such processors. Alternatively, or as acomplement, the position correction map could be forwarded to theposition servos 1205 controlling e.g. the support table supporting themask substrate during the writing process. In this case, the correctionmap implies a correction of the position control for the servo system,and thereby the position of the pattern on the substrate. Alternatively,or as a complement, the size correction map could be forwarded to anexposure dose control 1207, such as a dose modulator. Hereby, acorrection of the dose according to the predicted correction map couldbe provided during the exposure.

There are in principle four driving forces for errors: the physicalproperties of the substrates on which the patterns are printed, i.e. themask blanks and the wafers or panels, the position on the substrate, theexposing equipment (which can be using electromagnetic radiation orparticle beams as the exposing medium) including exposing sequence andenvironment, and the pattern itself. These driving forces interact witheach other directly of via the production processes to create errors. Animportant feature of the invention is the use of models to translatemeasured or previously known physical parameters to a placement, size orshape error to be corrected. An example is a plasma etcher. The plasmais non-uniform towards the edges of the substrate and in local areaswhere the surface exposed to the plasma is different from other placesin the pattern. This creates a position- and pattern-dependent sizeerror. It can however be characterised with a small number ofparameters, such as an edge fall-off magnitude and typical length, and asensitivity and disturbance length for pattern variations. Using thismodel with four parameters it is possible not only to precompensate forthe edge fall-off, which is equal from plate to plate, but also for thelocal variation which vary with the patterns.

Using a model-based error prediction makes it possible to account for alarge number of different error mechanisms with a manageable amount ofempirical data collection. Designing sampling and measurement plans thateffectively fit the parameters of a model from a limited number ofmeasurements is known in the art, and can be found in textbooks onexperimental design. It is also known how to design plans to separatebetween different driving forces. The ideal situation is thatmeasurement are made non-destructively on production masks and wafers,but for a specific model it may be more efficient to use a specialmonitor substrate, i.e. to be able to distribute a matrix of teststructures over the entire surface.

The model-based error prediction is further described in FIG. 6. Twodifferent mechanisms for generation of errors are set up, e.g. edgefalloff and pattern dependent etch activity in an etch step. Ameasurement is designed which can find the parameters for each andseparate between them. In this example it can be measuring features inareas with three densities each at three locations, inside,intermediate, and close to the edge of the substrate. The measurement isdone and the parameters extracted. Before writing and correcting apattern relevant information has to be collected in this case thepattern density in different areas of the pattern and the distance ofpattern areas to the edge of the substrate. The total correction isgenerated, in most cases by superposition of the corrections for the twomechanisms, other times by a more complicated summation.

Another aspect of model-based error correction is shown in FIG. 8,namely decomposition of complicated error behaviour into a set ofindependent and computable error mechanisms. The general geometricalerror in a mask writer is very complicated, but it can be decomposedinto isotropic expansion, built-in shape, gravitation sag, clampingdeformation due to non-ideal geometry of the clamping structure, andinteraction between the built-in shape and the clamping geometry. Ifeach error is small, which is the case for lithographic substrates, thecontributions can be superposed. On top of it come the stage errors,i.e. errors in the coordinate system of the writer.

An important case of model-based error correction is when a chip has aCD error towards the edge, for example due to stray light. If a 0.18feature is 3% percent too small one would expect that a +3% sizecorrection in the mask would compensate it perfectly. However, due tothe finite resolution of a stepper there is a more complicated functionrelating size on the mask to size on the wafer, the so-called “MaskError Enhancement Factor”, MEEF. The enhancement factor is sizedependent and depends on the details of the tools and process. Therefora model need to be used that takes the MEEF into account, and thecorrections will not be correct until the MEEF model has been verified acouple of turns around the feedback loop.

Process errors 502, if they can be held constant, can in principle becorrected with the feedback loop. Other errors 501 are impossible tocorrect by feed-back, because they are not constant in time. In theinvention one important such error has been identified as the clampingdistortion of the mask in the mask writer, the metrology system and inthe exposure station that uses the masks. At first sight the clampingerrors seem uncontrollable, but we have found that they can be predictedfrom accurate geometrical data for the mask blank itself and theclamping structure of equipment using or operating on the mask. Anotherseemingly random error source that is controlled in another embodimentof the invention is variations in linewidth due to variations in resistand chrome properties over the mask substrate. Using the invention it ispossible to set up and apply models for how the resist thickness andchrome properties affect the feature size of the pattern and correct forthe errors created.

Since these properties are possible to measure by measuring means 513,514 prior to the writing of the mask, it is possible to predict theerrors and correct them at the time of writing the pattern, thefeed-forward correction loop 515 in FIG. 5.

Residual Errors

By the feed-forward or feed-back correction 509, or especially acombination of the two, a large portion of the total errors can becontrolled and corrected. The residual errors are due to random errorsduring maskwriting, exposure and processing, and have to be addressed assuch, i.e. by better temperature control, vibration-insulation, processautomation, etc. They are also due to incomplete error models anduncertainty in the model parameters. We believe that when the frameworkof the invention is established there will be a development of betterand better models until eventually all but the genuine noise errors areremoved. The development of models and software for theircharacterisation and use could well become the mission of independentcommercial companies.

A flow chart describing the development of models and how the models areused to predict and correct errors is given in FIG. 6.

A Comprehensive List of Error Sources

FIG. 7 shows the work flow from glass block and CAD file to a finishedchip or TFT display, and important error mechanisms that can be modelledand corrected. The work flow is divided in three separate parts ofmaking the mask blank, writing the mask on the blank, and finally usingthe mask for lithographic production, and each part is divided inseveral different steps. For each step is further indicated differenttypes of errors possible to occur during said step. These errors arehowever merely examples of errors possible to occur during each step,and many other errors are probably possible as well. As is describedabove, errors in at least some of the steps are measured and used eitherin a feedback or a feed-forward loop to predict the error in the writtenpattern, and to generate corrections to compensate for said predictederrors during the exposure step in the mask making.

Correction of Clamping Errors

If is held in exactly the same way during writing and use there is noerror. Until now it has been possible to treat the semiconductor mask asa stiff plate with no deformation induced by clamping, provided that theclamping has been done carefully. There are two developments that makethe clamping-induced deformation more critical in the future: increasedmask size and dramatically tightened precision requirements. The newmask format of 225×225×9 mm has been defined and the allowablegeometrical errors in a mask will be 30 nm in 2001 and below 10 nm a fewyears later. The registration error allowed is typically 5% of thefeatures on the photomask and the error of critical dimensions (CD) mustbe less than 2.5%. Current plans for lithography predict that thefeatures will be around 25 nm in the year 2010. With a reduction rationof 4 the features are 100 nm on the mask and the registrationrequirement is then 5 nm and the CD tolerance 2.5 nm. A few years laterthe requirements are predicted to be sub-nanometric if the march towardsmaller scale were to continue. Mask production following what is knownin prior art cannot produce masks to these requirements. In theinvention methods are devised that can reduce many systematic errors byan order of magnitude or more.

It is known in prior art to support the glass plate at three points.With three supports there is no bending induced by the supports.Therefore it deforms only under gravity. It is also known to correct thepattern geometry for the computed deformation due to the gravitationalsag when it is supported on three supports. The deformation depends onlyon the plate size and its material properties and can be computedbeforehand.

In flat panel production the masks may be 600×800×12 mm. It is notpossible to support such a large plate on three points and get asufficiently flat surface. The plate needs to be supported at more thanthree points and becomes kinematically over-constrained. The supportingstructure will introduce deformations if the points are not in a perfectplane. In this case the deformation is a combination of gravitationalsag and deformation due to the support structure. The same applies to aco-ordinate measuring machine where the plate needs to be measuredwithout distorsion.

The basic problem is that due to technical constraints differentmachines producing and using a patterned workpiece hold the workpiece bydifferent methods, For example a reflection-type metrology system forsemiconductor reticles normally uses a three-point support with thesupport points chosen for minimum deformation, but the stepper using thereticle must support it along the edges to keep the patterned areaunobstructed for the exposing light.

If the workpiece or the clamping structure is non-ideal, i.e. non-flat,the different types of clamping in different types of equipment give anuncertainty in the geometry of the workpiece.

A numerical example: assume that a standard semiconductor reticle(152×152×6.25 mm) is supported at the four corners. One of the cornersis 1 μm out of the plane of the other three corners, either because theglass is non-flat or the supporting points are skewed. The bending ofthe glass causes a line along one diagonal of the plate to be stretchedand the other diagonal to be compressed. This is the same as anorthogonality error in the pattern on the mask with a maximum lateralposition error on the plate of 0.5 μm*6.25/2 mm/152 mm=20 nm. The errorfrom the clamping must be added to other error sources such as drift,scale errors and effects from the process. Therefore 20 nm isunacceptable as an error from the clamping alone. And if the points werelocated closer together or if there were more than four constrainingpoints the errors would be even larger. We have found that supporting aplate on four well chosen points is exceptionally good for large plates,giving a deflection that is 20 smaller than with three support points.The invention makes it possible to use four points and compute theeffect of the plate being over-constrained. If the flatness inforce-free state is known and the height of the upper surface ismeasured at the four point one has all necessary information to correctthe pattern for bending and gravitation.

The invention devises a method for complete prediction of the clampingerrors as well as partial correction for stress-induced errors in amulti-machine environment with equipment for writing masks, printingpanels or wafers from masks and metrology systems for measuring masks,wafers and panels.

DESCRIPTION OF A PREFERRED EMBODIMENT

The mask blanks are cut and polished by a glassmaking company. Thesurface figure is controlled to a maximum error corresponding to aquality class of the glass product. The glass plate is coated with asputtered film of chromium and a photoresist coating is spun on. In thepreferred embodiment of the invention the flatness of the front andbackside of the glass is measured before and after the coating withchrome and after the resist coating. A flatness map is generated foreach side together with other auxiliary information such as the exactthickness and the Young's modulus of the glass material. Each glassplate has a serial number engraved at the perimeter of the chromesurface before the chrome coating so that the identity of the plate canbe tracked through-out it's lifetime. The serial number is engraved inthe glass surface or chrome film in clear text and machine-readableformat, e.g. by laser ablation. It is also possible to laser engrave anidentity mark inside the volume of the glass or use other markingmethods such as magnetic recording in the chrome coating or embedding ofa memory devise in the glass plate. Identification of semiconductorwafers by an engraved marking is standard in the semiconductor business,and in the invention the same would be applied to mask blanks. Any othersecure identification system can be used, for example storing andshipping the mask blanks in marked and bar-code labelled boxes.

The mask blank maker stores the flatness data for each blank on acomputer and the data is published on a network accessible to the maskmakers at a later time, e.g. on an Internet server. Alternatively thedata could follow the blank, e.g. on a diskette shipped with the blankor on the embedded memory device.

How to measure the flatness and other properties of the mask blank isknown in the art. Flatness is often measured by interferometers made byfor example the companies Möller-Wedel, Zygo and Tropel. The resistthickness can be measured with spectroscopic reflectometry, ellipsometryand other optical methods. The chrome thickness can be measured byoptical transmission or inductive methods and the reflectivity of thechrome layer by reflectometry. The exact thickness can be measured withmechanical or optical methods.

When a mask or reticle is ordered by a semiconductor company thespecification includes the serial number of the stepper for which themask is written. The clamping system of each stepper is characterised bya file of geometrical data describing the method of clamping, but alsoand most important the individual imperfections of the clamping system.This information is stored on a network accessible to the mask maker orsent to the mask maker together with the order. The mask maker can alsopull information about the process, e.g. uniformity data for a etchingstep from the semiconductor manufacturers computer. Alternatively thedata can be attached to the order document as embedded data or asseparate documents.

The mask maker who typically have several mask writers of different kindhave a similar database of data for his writing systems and processes.He also has empirical data of how the mask is distorted by theapplication of a pellicle, the stretched film that acts as a dustprotection on the finished mask.

When the chrome and resist were applied at the mask blank manufacturerthe blank bent due to stress built into the films. When the resist isremoved during processing in the mask shop the stress from the resistdisappears and the blank goes back to the state it was before the resistwas applied. More important is that when a pattern is formed by partialremoval of the chrome the stress from the chrome film is partiallyrelieved. With knowledge of the bending caused by the application of thechrome film during manufacture and the pattern to be written it ispossible to predict the deformation of the plate by patterning of thechrome.

During the planning of the writing job the mask maker fetches theapplicable information for the stepper or exposure station in which themask will be used, for the maskwriting equipment, for the blank and forthe pattern. Using the error model a total combined error can becomputed and corrected for, optionally using the MEEF factor or asimilar function as a transfer function from correction to result in thefinished pattern. In another application of the invention the collectedinformation can be used to select among mask blanks, writing systems andprocesses. A simple example is the selection of blanks for uniformityfor patterns that occupy different areas on the mask. Of course, theprinciple of the invention is the same whether the end result (“theoutput”) is a chip, a display panel or just the mask.

This contains geometrical information about the clamping geometry,imperfections of the clamping structure and also other known imagedistorsion created by the tool and subsequent processing.

The Maskwriter

In a preferred embodiment the mask writer has several provisions forcorrection of distortions caused by the processes chain from patterningto use. First it has a precision stage controlled by laserinterferometers with a adjustable scales in x and y, corresponding touniform shrinkage or expansion plus uniform bending across each axis,such as may result from built-in film stress. Secondly it hasadjustments for orthogonality and trapezoidal distorsion, bymodification of the driving of the servos and by software. For higherorder errors such as barrel distorsion, mirror bow and irregularity ofthe coordinate system, the maskwriting machine has an xy correction mapthat sends position-dependent offsets to the servo systems. Theinformation collection and error prediction system sends a uniform scaleand an xy map to the maskwriter for each mask to be written, said mapbeing the pattern correction necessary to correct for all known errors.These of course include the maskwriter's own stage errors.

Furthermore the maskwriter has a clamping structure that is adapted tothe particular type of mask blank to be written. In one embodiment theplate is placed on three supports so that the deformation of the platein the maskwriter is independent of the plate shape and can be computedeasily. For the most accurate correction the supports are placedidentically to similar supports in the equipment using the mask, e.g.near two adjacent corners and at the center of the opposite side.

In many cases it is not possible to support the mask plate at threepoints. Especially for larger-size masks it is necessary to have morethan three supports. In this case it is impossible to hold a maskwithout introducing bending moments, if not both the support structureand the plate are perfectly flat. In one embodiment the forces exertedon the supports from an ideal plate/support structure combination arederived theoretically and the geometry of the supports is adjusted untilthe force on each support matches the theoretical force. In anotherembodiment the non-flatness of the plate is known beforehand and thegeometry of the clamping structure is modified so as to minimise bendingforces. Other deformations such as that from gravitational sag andsub-sequent process distortion are computed and corrected for.

In another embodiment the support structure is not adjustable butgeometrically characterised, so that it is possible to compute thebending of a particular plate resulting from the combination of platenon-flatness and clamping geometry.

In another embodiment the maskwriting system itself is adapted tomeasure parameters that are needed for the correction, such as theflatness of the substrate. If the flatness is measured after clamping inthe maskwriter, the measured data can be used in several ways. First itcan be used to check the model of the clamping structure. Secondly itcan replace the detailed knowledge of the stage flatness, since theexact form and the clamping deformation can be calculated from thesurface flatness combined with flatness data of the substrate. In thecase that there is no flatness data available and the plate is known tobe flatter than the stage and/or the gravitational sag, the measureddeviation from flatness indicates a real deformation and can be used foran approximate correction.

A measurement of other parameters such as resist thickness can also beintegrated with the maskwriter, to provide information necessary for thecorrection independent of the mask blank maker or to be used within-house spun plates.

The invention can be used in different types of maskwriters usingscanning laser beams, spatial light modulators or particle beams.

Exposure Station

Like the maskwriting equipment the exposure station or stepper that isgoing to use the mask to print the pattern on a workpiece has a numberof errors, such as image distorsion in the exposure step and warpage inthe subsequent processes. In a preferred embodiment the mask issupported by three supports and the clamping gives no additional bendingforces beside the gravitational sag. In another preferred embodimentother design constraints makes it necessary to clamp it kinematicallyover-restrained, i.e with more than three support points. The patterndistorsion due to the bending forces resulting from imperfect geometrycan be cancelled by modification of the clamping structure as in themaskwriter or by pre-diction and precorrection of the mask.

The geometry and the errors of the exposure station are characterisedand stored as a machine parameter file, which contains enoughinformation to compute the errors of a real physical mask and how it isprinted. The file may, apart from the identity and bookkeepinginformation, contain, the number of supports, their xyz coordinates andcompliances and optional springloading, further distorsions of themachine's coordninate system etc.

It is also valuable to have a process distorsion file which contains anerror map for how an exposed workpiece is distorted by subsequentprocessing. An example is a glass panel for a TFT-LCD display that mayshrink by several tens of ppm in high-temperature steps.

Metrology Tools

The masks and the exposed workpieces are measured in coordinatemeasuring system, such as those commercially available from thecompanies Leica and Nikon. The metrology tools also have a clampinggeometry and built-in errors that can be described in a machine geometryfile. Even though the metrology tool does not impose its errors directlyto the mask, it does so indirectly by being the reference against whichall other systems are calibrated.

Computing the Bending Forces

A mask blank has a simple geometry and is made from high-quality quartsor glass. Therefore, it can be represented by a simple finite elementmodel as is well known to a person skilled in mechanical design. Allerrors are small compared to the size of the plate and the resultingerrors can be computed by linear superposition of distorsions fromdifferent sources, e.g. gravitational sag and bending due to clampingare additive. This allows for simplified methods of analysing theelastic glass plate by decomposition of different bending modes. This isadvantageous for real-time correction on an embedded computer in themask-writer, but it is equally possible to run a full finite elementsimulation on an embedded computer with adequate memory and power, or atan offline workstation.

Given a machine parameter file, with the geometry of the supports andthe orientation of the force of gravity relative to the plate, and aplate parameter file with the geometry, flatness and elastic propertiesof the plate, the computer can use the finite element model to find theshape of the plate, the distorsion of a pattern on its surface, and thecontact forces at the support points.

If there are many support points it is not known beforehand that aparticular non-flat plate really makes contact to all supports. It is,however, possible to find which points make contact. In principle thesolution has to be self-consistent. The plate makes contact to a supportpoint if there is a positive contact force, including the effect ofpossible spring-loading. By this method it is also possible to find theapproximate area of contact between the mask blank and a flat surface,e.g. a flat stage top, by representing the flat surface by an array ofcontact points.

Because of the linearity of the elastic plate and the simple geometry itis also possible to derive a simplified set of simultaneous linearequations describing the deformation of the workpiece due to forces at anumber of predefined positions on the workpiece. Other positions can betreated by interpolation between the basic computational points. Thesystem of equation is solved for the geometrical constraints given inthe geometry files and the bending forces are derived. These are addedto the gravitational sag that depends only on where the support pointsare placed.

Correction of Other Mask Blank Related Errors

Other properties of the mask blank affect the quality of the image, suchas the resist thickness, the chrome thickness and the reflectivity ofthe chrome. These can be measured in the same way as the flatness or infact in the same equipment.

Implementation of the Correction

Once the errors are predicted and the appropriate corrections arecomputed there are two principally different methods to apply them.Either the pattern data is modified, e.g. the corner points of eachfeature are moved, or else the corrections are fed to the writinghardware, such as position servos for placement and modulator or lightsource for intensity control. The former method is more general and canin principle give arbitrary large corrections. It is also the onlypossible alternative for shape corrections involving serifs and similarfeatures. For small and slowly varying corrections the second methodgives a smoother correction, since it has the resolution of theinterferometer while modification of the data only has the resolution ofthe address grid. A second advantage is that the data can be preparedoffline without knowledge of the mask blank and machine specifics andthe corrections applied at the time of writing. But for very densepatterns the data preparation has to be done in real time anyway, sothere is little difference in logistics whether the correction isapplied to the pattern or to the writing hardware. Correcting thepattern is perfectly flexible: corrections can be applied for placement,size and shape and there is no limit to the size or complexity of thecorrections.

Real-Time Pattern Correction

With a real-time data path with high processing capacity it is possibleand advantageous to apply corrections for beam size or resolutioneffects and stray exposure together with all other corrections at thetime of writing. In a typical implementation there is a bank of parallelprocessors, possibly organised in groups doing different steps of thedata preparation, with up to several hundred CPUs and ASIC:s. The slowlyvarying corrections, e.g. for clamping distorsion, will not effect thework load or data flow appreciably, but corrections for resolution, beamsize and stray effects generate immense amounts of extra data andrequire processing capacity to match the data flow. However, there is anbig advantage to real-time correction and for exactly this reason. Thedata volumes are difficult to handle with off-line correaction. In acommercial mask shop where a maskwriter is running essentially 24 hoursa day, there is also no cost benefit of doing the correction off-line,since an equally powerful computer is needed to furnish corrected datato the maskwriter at full writing capacity.

The general concept of pattern correction to compensate for the limitedresolution is known in the prior art. In the invention the correction isdone in real time as a part of the real-time processing. For real-timecorreaction of resolution, beam size and stray effects the algorithm ispreferably running on the maskwriter's embedded processor bank. Thecontrol system supplies only the parameters for the interaction betweenthe image formation and the pattern, e.g. a series of superposedgaussian profiles representing the point-spread function or the beam ofthe writer and exposure system, or a system of rules for the correction.A typical rule is to add a 0.16 □m serif to all outer corners in thepattern.

Correction of the imaging properties of the maskwriter is preferablydone transparently, with parameters that are fitted to that particularmaskwriter and process but otherwise not changing. Once the parametersare set up the user will not need to care, or even know, about thecorrection, but will only see a mask pattern with a more accuraterepresentation of the input data.

Pattern correction for the exposure station is preferably done incollaboration between the mask shop and the chip or panel manufacturer.The manufacturer prints some test patterns that are designed forextraction of the model parameters. Once the model parameters are setthe correction could be transparent and automatically applied to alldesigns. The information system of the invention pulls the parameterstogether with clamping data and image distorsion when the writing job isset up. Or alternatively the manufacturer would explicitly provideanother set of correction parameters, or correct the data directly usinghis own correction model. However, we believe that the design of imagecorrection models, design of test patterns and extraction software willbe the mission of specialist consulting companies, and that themanufacturer will use the system in the transparent mode. Theinformation system of the invention provides the framework, which makessuch transparent operation possible and convenient.

FIG. 9 shows a typical implementation of an error correction systemsuitable for the invention using both pattern modification and writinghardware control. The writing hardware 901 prints a pattern 902 on themask blank 903 using a low-level representation of the pattern 904, e.g.a bitmap or a decomposition into small area elementary forms such astrapezoids. The low-level format is created from a geometrical databasewith a high-level input 905, containing a geometrical description of thepattern, such as a list of filled polygons. Since the input format cancontain the geometrical features in any order and they can have anyshape they are pre-processed into an intermediate format 906 afterhaving been distributed 907 from the external file interface 913 on anumber of parallel processors 908. For each conversion step the datavolume expands and the necessary processing power increases. Thereforethe final processing needs more parallel processing units 909 than thepreprocessing. The error correction system has a control unit thatpredicts the errors from the collected error data and computes anappropriate pre-compensation based on its set of models and rules. Thecorrection of small slowly varying size and placement errors is sent 912to the writing hardware, especially to the dose control and the positionservos. Pattern corrections are sent in the form the correction rules tothe datapath interface 913 and run on the embedded computer banks 908and 909. Even the pattern corrections can take different forms andseveral correction algorithms can be run at different conversion steps.For example, correction for the beam size effects in the mask generatoris suitable to run at the last stage of the conversion, while the largerserifs needed to compensate for the imaging effects in the stepper areadded to the pattern during the preprocessing step.

For efficiency it is suitable for the different processors to work onseparate areas of the pattern, but to apply corrections for non-localimaging effect the processor needs to know the pattern in an area aroundthe actual point it is working in. Therefore the pattern is cut intopartly overlapping computation fields. The redundant information is onlyused for the correction and discarded after use. For faster processingof non-local information, e.g. for correction of stray exposure, atemporary representation of the pattern at a lower resolution is createdand used to compute background exposure. It is possible to use more thanone low-resolution representation at a single time to representphenomena at different length scales.

In one embodiment the two processing steps are complemented with a thirdstep that runs on a separate bank of processors essentially dedicated topattern correction.

In a preferred embodiment of the invention two correaction maps arecomputed, one for position and one size.

The position map has the form of a table with x and y deviations givenat a grid of points. For a given stage position the corresponding s andy correction is computed by interpolation in the correction map and theresult is fed as an offset to the position servos. The same method isused for finding the correction of CD errors by interpolation in the CDmap.

In a different embodiment the correction maps are computed and madeavailable to the datapath. During the conversion from pattern databaseto hardware-driving signals both position and size is corrected. In thepreferred embodiment, which has a stripe-organised writing strategy, itis done at the fracture step, just before the pattern is divided intostrips. The vortex points of the pattern elements are simply movedaccording to the interpolated correction maps. Most pattern elementshave edges parallel to the axises, and would after the correction haveslanted lines, but since the scale of the correactions is normally ofthe order of a part per million or less, all but a few edges are stillparallel to the axises after their coordinates have been truncated tothe resolution of the data preparation software, e.g. 0.1 nm. Beforewriting they are further truncated to the address grid of the writer,e.g. 4 nm for semiconductor reticle at a specific specification level.The written pattern will have the smooth curves of the correction mapsnapping to the address grid, but with an appropriately chosen addressgrid this will give a negligible contribution to the error statistics.

The Information System

An information system is built to manage the lithography errors in thechain from mask blank maker to user of the final products. The hub inthe information system is the mask maker where during job planning allinformation is collected and used to predict errors and pre-compensatethem.

A convenient way to organise the information when several parties areinvolved, e.g. mask blank makers, mask shops and mask users, is to haveeach party maintain its own information and store it on its own computersystem. The computers are accessible from the mask maker's computer byremote access over phone line, ISDN, high-speed link or the Internet. Inthe latter case it is important that the integrity of the information isvalidated, so that the user of information knows it is complete,unaltered and issued by the correct sender. The parties involved mayalso want to have it confidential. All this can be assured in moderncommunication links by appropriate use of passwords, encryption, checkfigures and digital signatures.

Workflow

It is the responsibility of the mask shop to find the necessarycorrections that will make a particular reticle print without systematicregistration errors. The customer, i.e. the mask user, provides one ormore pattern files and the order document requests any specialtreatment, metrology etc. In a preferred embodiment of the invention theorder document specifies the exposure station where the mask will beused and optionally also a process error description. During jobplanning and set-up the mask maker's computer accesses the data from themask user, data which specifies the geometry of the exposure station,including known imperfections, and image distorsions. Optionally themachine file also includes a map of size errors created by the exposurestep. If a process file has been specified in the order document, itspecifies warping of the workpiece during subsequent processing andoptionally also size errors created by the process. For example a plasmaetching step is sensitive to the “loading”, i.e. the local density ofpattern area exposed to the plasma, and etches differently along theedges of a chip. Since this is a systematic behaviour that is repeatedmore or less in every design it can be partly corrected for in the maskusing information about said mask.

When the writing job is planned a mask blank is assigned to the mask.The computer accesses the data storage at the mask blank maker or alocal storage with information files for the blanks that are in stock.The information file fetched corresponds to the particular mask blankthat will be used for writing the mask, and holds information about theexact size and thickness as well as relevant physical materialproperties. It also contains flatness data, resist thickness, chromethickness and reflectivity and data relating to the bending forcecreated by built-in stress in the chrome and resist. The scheme is, ofcourse, independent of the exact materials used, and other types ofexisting or future mask blanks can be handled in a similar fashion.

Data File Formats

The data file formats are designed to be extendable by having the datafields tagged with keywords. A new feature can be included afterdefinition of a new keyword and old data files will still be compatible.For simplicity the data files in the preferred embodiment are ASCIIcharacter files. This allows for simple debugging and files can bemodified or created in any text editor or spread-sheet program. To avoidthe risk of inadvertent change of a file it is locked by a checksum andvalidated by a digital signature. For machine-generated files these aregenerated automatically, but hand edited files need to get the checksumand signature added by a special validation program. This gives areasonable trade-off between security and flexible engineering anddebugging. For encryption any commercial encryption program can be used.

Example of a System Using an Embodiment of the Inventive Method

A system for producing large-area displays with distortion controlaccording to an embodiment of the invention should now be described morethoroughly.

Referring to FIG. 10, an embodiment of the system for producing largearea display panels according to the invention is shown. The systemcould be used for producing shadow masks for conventional CRT (CathodeRay Tube) displays, but is especially useful for producing TFT (ThinFilm Transistor), CF (Color Filter), PDP (Plasma Display Panel) or PALC(Plasma-addressed liquid crystal) displays.

The system comprises a first mask producing means 1001 for producing amask with a predetermined pattern according to input data. The maskproducing means is preferably a microlithographic writing device forwriting with high precision on photosensitive substrates. The termwriting should be understood in a broad sense, meaning exposure ofphotoresist and photographic emulsion, but also the action of light onother light sensitive media such as dry-process paper, by ablation orchemical processes activated by light or heat. Light is not limited tomean visible light, but a wide range of wavelengths from infrared toextreme UV. Such a mask producing apparatus is previously known frome.g. EP 0 467 076 by the same applicant. In general the apparatuscomprises a light source, such as a laser, a first lens to contract thelight beams, a modulator to produce the desired pattern to be written,the modulator being controlled according to input data, a reflectingmirror to direct the beams towards the substrate, and a lens to contractthe beams before it reaches the substrate. The mirror is used for thescanning operation to sweep the beam along scan lines on the substrate.Instead of a mirror, other scanning means may be used, such as arotating polygon, rotating prism, rotating hologram, an acousto-opticdeflector, an electro-optic deflector, a galvanometer or any similardevice. It is also possible to use raster scanning or spatial lightmodulators. Further, the substrate is preferably arranged on an objecttable which has a motion in two orthogonal directions relative to theoptical writing system, by means of two electrical servo motors.

The system according to the invention further comprisesmicrolithographic exposing means 1002 for exposing a photosensitivepanel substrate with light and with use of the mask to impose thepattern of the mask on the substrate, whereby said substrate has a layerbeing sensitive to said light. Several such exposing means are alsopreviously known in the art. The exposing means could be of the contactcopy type, proximity exposure type, or a projection aligner. The systemaccording to the invention could also be used in a direct writer,whereby the compensation is not made in a physical mask, but in a datamapping controlling the writing beam. For TFT and CF display panelsprojection aligners are usually used, and for PDP and PALC the contactor proximity type are frequently used.

Furthermore, the system comprises measuring means 1003 for measuring thepattern on the substrate and detecting deviations relative to theintended pattern as given by the input data. This could be done bymeasuring the geometrical position of the pattern, preferably at somereference positions, to get a so called registration mapping, andcompare it with the intended pattern which is deducible from the inputdata. Further, the width of lines in the pattern, the so called CD(Critical dimension), could be measured. Measuring equipment iscommercially available, and for example the equipment could comprise aCCD-camera or be based on interferometry.

From the measuring means 1003 a distortion control signal is sent to asecond mask producing means 1004. This second mask producing means couldbe a separate apparatus, but is preferably the same as the first maskproducing means 1001. This second mask producing means is fed with inputdata describing the intended mask pattern to be written, and is also fedwith the distortion control signal from the measuring means 1003,whereby the writing process for producing the second mask is controlledto modify the pattern to compensate for the measured deviations, andthus compensate for production distortions. The measurement ispreferably made after the subsequent processing steps of the panel aswell, i.e. the development, blasting and/or etching, whereby systematicerrors from theses processes are taken care of in the compensation aswell.

The compensation in the mask writer could be accomplished in differentways. In a writer of the type described above, with an object tablecontinuously moving in a slow strip direction and a scanner sweeping ina fast scanning direction, the compensation could be made according to asurface mapping. According to this mapping the compensation in thescanning direction could be accomplished by e.g. offsetting the startingtime of the beam during the scanning. In the stripe direction thecompensation could also be made by time offsets, either directly orindirectly by means of different ramp functions. There are also otherpossible way to accomplish such compensation. For example thecompensation could be made by controlling the servo motors for theobject table, by adjustment of the time dependent angle of the scanners,by changing the input data or by controlling an internal control unitsuch as piezoelectrically controlled mirrors.

However, if a direct writer is used, the same type of compensation couldbe made in real time.

Compensation for deviations in the line width, CD, could be accomplishedin the same way as deviations in the registration. However, thiscompensation could also be made by changes in the power of the writingbeam, i.e. the exposing dose, by changing the laser output or having ananalog modulator. This compensation could be accomplished by means of aherefore adapted dose mapping to control the dose.

When the second mask is used in the same exposing means 1002 allsystematic errors depending on different temperature conditions, errorsin the exposing means etc., are compensated for, and the patternprecision of the produced display panels are greatly improved.

The first mask could either contain the same pattern as the intendedpattern for the second mask, i.e. the pattern not being compensated, orcontain a reference pattern, intended for deviation measure only.

Further, error data could be accumulated, and a rolling means valuecould be used for the compensation. The error compensation could also bea combination of several different part error compensations. Those partcompensations could be based on the premises for the process, e.g. whichstepper and type of glass that is being used. Hereby the total errorcompensation could be a combination of one or several errorcompensations for each process step.

Above, a system for passive distortion control has been described. Inthis system compensation is made for the processes and equipment beingused in the system. However, the compensation is not adapted fordifferent panel substrates. In this passive system a measurement toalter the distortion compensation is preferably made once for every newbatch of substrates, and thereafter the same mask is used for producingall the panels in the batch. This passive distortion control isspecifically useful for production of TFT or CF displays. The requestedprecision for the patterns on the mask for this production is extremelyhigh, and the masks are very difficult, and thereby expensive, tomanufacture. On the other hand the masks last for a long time in thisproduction.

The system according to the invention could also comprise secondmeasuring means 1004 for measuring the thickness of the light sensitivelayer on the substrate prior to the exposure, whereby said measurementis also used for said compensation. Hereby the compensation is adaptedfor varying resist layers between different batches of substrates. Suchbatch wise compensation could also be accomplished with use of dataspecified by the manufacturer.

This second measuring means 1004 could also be used for measuring eachand every panel substrate that is going to be exposed, and thereafteradapt the process for each individual panel. Hereby the system couldcompensate for varying glass quality in different panels, varyingthickness and quality of the resist or emulsion of the substrate area,different form variations etc. This active distortion control isespecially useful for production of PDP or PALC display panels, wherethe masks are comparably easy and inexpensive to produce. This methodcould also be used for direct writers.

In the active distortion control the panel is initially measured,regarding e.g. resist thickness. Many such measuring methods areavailable for someone skilled in the art, e.g. a test exposure,dosimetry, of the substrate with different doses, by profilometry,interferometry, confocal microscopy, by an interferometric method or thelike. The shape of the substrate could also be initially measured, andthis could be accomplished by known methods such as moiréinterferometry, projected fringes, laser triangulation, ordinaryinterferometry etc. Preferably already existing patterns are alsoinitially measured, whereas such exists. Display panels are usuallyexposed in several separate steps, typically 3-7 exposing steps, andnormally the same exposing station is used for all the exposures. Bywriting masks with compensation for individual errors in differentstation the display producer could schedule the production more freely,independent of which stations that is used. This is of great importancefor making the production more efficient and the utilisation of thestations better.

Referring now to FIG. 11, a method for producing large area displaypanels according to the invention, and with use of the above-mentionedsystem will be described.

The method according to the invention comprises a first step S1 in whicha mask with a predetermined pattern according to input data is produced.Thereafter the mask is used for microlithographically exposing aphotosensitive substrate with light to impose the pattern of the mask onthe substrate, whereby said substrate has a layer being sensitive tosaid light, in step S2. The exposed pattern is then measured, possiblyafter several subsequent processing steps, or even in the finishedproduct, in S3, to detect deviations of the exposed pattern relative tothe intended pattern as given by the input data. In step S4 a distortioncontrol mapping is then produced, to be used in step S5 duringproduction of a second mask having a pattern according to input data andmodified to diminish the measured deviations, and thus to compensate forproduction distortions. In the last step S6 the second modified mask isthen used in a photolithographic fabrication of display panels. Similarcompensation may be used in a direct writer, where the compensationcould be made in a data mapping.

Dose Compensation

CD uniformity may be an important specification for photo masks. A finalresult may be a combination of pattern generator equipment uniformityand process equipment uniformity.

After exposure of a mask in a pattern generator the resist covered platemay be processed in different process systems, e.g., hot plate,development equipment for resist, and etching equipment for chrome. Theprocess equipment steps may introduce systematic CD variations on theplate, e.g., cylindrical symmetry, due to the physical design of processequipment.

If the CD contribution from process equipment is stable over time theoverall result of CD uniformity on the final mask may be improved byintroducing a CD correaction of opposite sign, (process compensation),in the pattern generator while printing the plate.

As described above in detail above, process compensation in a patterngenerator may be generated directly in the data path used for patterngeneration by modifying the printed pattern itself. As also describedabove, alternatively, process compensation in a pattern generator may begenerated indirectly by changing other global parameters as, e.g.,exposure dose while printing the plate. For example, CD may be afunction of dose.

Writing Principle, SLM Writer

An SLM writer may operate as a DUV stepper with a programmable mask.Hence, a full image may be generated as series of 2D projected SLMstamps, where each SLM stamp may be printed with a separate laser flashof short duration. Multi pass printing including offset between passesmay be used that substantially reduce systematic CD errors over the SLMstamp.

Global CD, may be corrected with a dose system, using a dose map, aseach individual SLM stamp is small compared to the full 6″ plate.

As shown in FIG. 13, a dose map as a function of xy-position on the maskmay be generated, e.g., by using the gray levels of SLM or by using apre-programmed global dose profile utilizing the dose control in thelaser. During the exposure cycle the x-stage may move at constant speed.As shown in FIG. 13, one or more interferometers may generate triggerpulses at correct positions and act as a master trigger in the system.

Both the data path/SLM and the laser may be controlled by this mastertrigger. Process compensation using a dose map(x,y) may be implementedusing the SLM or laser dose control.

FIG. 14 a illustrates an example of a (2D) cylindrical CD distributionaccording to an embodiment of the invention. As shown in FIG. 14 a, nodose compensation is provided.

FIG. 14 b illustrates an example of cylindrical CD distribution. Asshown in FIG. 14 b, dose compensation is provided and CD is compensated.

FIG. 14 c illustrates an example of cylindrical CD distribution usingglobal dose compensation. As shown in FIG. 14 b, dose compensation isprovided and CD is compensated.

FIG. 14 d illustrates an example of a pre-programmed global dose mapimplementable using gray levels of the SLM or by utilizing dose controlin the laser.

As set forth above, example embodiments of the present invention mayimprove precision by predicting and/or measuring errors and correctingthe errors. As also set forth above, example embodiments of the presentinvention may improve precision by compensating directly in the datapath used for pattern generation and/or compensating indirectly bychanging other global parameters, such as, e.g., exposure dose. As alsoset forth above, example embodiments of the present invention mayimprove precision by maintaining correction information in a correctionmap or correction profile.

The invention shall not be limited to the embodiments described above,which are only examples of implementation. For example what is saidabove about chrome-on-glass masks is equally applicable to masks madefrom other materials, e.g. with coatings of other metals, iron oxide,diamond-like carbon, multilayer coatings etc., and with other substratematerials such as calcium fluoride, Zerodur, silicon etc, as well as forboth transmissive and reflecting masks and masks which work byscattering. It is also applicable to stencil masks and membrane masksfor particle-beam and x-ray lithography.

1.-7. (canceled)
 8. A method to compensate for process variations whenprinting a pattern on a workpiece, said method comprising: determining atwo-dimensional CD profile in said pattern printed on said workpiece,generating a two-dimensional dose compensation profile to equalizefluctuations in said two-dimensional CD-profile, and patterning aworkpiece with said two-dimensional dose compensation profile.
 9. Amethod to compensate for process variations when printing a pattern on aworkpiece, said method comprising: predicting a two-dimensional CDprofile in said pattern to be printed on said workpiece, generating atwo-dimensional dose compensation profile to equalize fluctuations insaid 2-dim CD-profile, patterning the workpiece with said 2-dim dosecompensation profile.
 10. An apparatus for process variationcompensation when printing a pattern on a workpiece, said apparatuscomprising: determining a two-dimensional CD distribution in saidpattern printed on said workpiece, generating a two-dimensionalcompensation file to equalize variations in said two-dimensional CDdistribution, and patterning a workpiece with said two-dimensionalcompensation file.
 11. A method of compensating for CD variations on aworkpiece while printing a pattern on said workpiece, said methodcomprising: determining two-dimensional critical dimension (CD)variations associated with said pattern being printed; estimating futureCD variations of said patterning process; compensating the dose whilepatterning said workpiece by pro-actively equalizing for said estimatedCD variations; and patterning said workpiece using said compensation ofthe exposure dose.
 12. An apparatus for compensating for CD variationson a workpiece while printing a pattern on said workpiece, saidapparatus comprising: means for determining two-dimensional criticaldimension (CD) variations associated with said pattern being printed;means for estimating future CD variations of said patterning process;means for compensating the dose while patterning said workpiece bypro-actively equalizing for said estimated CD variations; and means forpatterning said workpiece using said compensation of the dose.